This invention relates to material handling equipment, and, more particularly, to a boom point sheave assembly for the outer end of a material handling boom, such as in a dragline excavator or crane.
Material handling machinery is commonly provided with a boom which is capable of moving in a vertical plane. Vertically oriented sheaves are mounted on the outer end of the boom and each sheave has a hoist rope reeved therearound. Additionally, the boom typically can swing from side to side, and as the boom is swung the rope and its load may lag behind the sheaves and develop a certain amount of "offlead" between the boom point sheaves and the suspended load. Each hoist rope then extends at an angle with respect to the plane of its corresponding sheave. This may result in severe frictional wear on the hoist rope and significantly reduce its useful life. Another consequence is that torsional stresses are imposed on the boom from the "offlead", or lagging of the load.
To reduce the frictional wear on the hoist ropes and torsional stresses incidental to the offlead situation, it is common to provide a boom point sheave assembly which is able to follow the hoist rope misalignment. A typical boom point sheave assembly designed for overcoming these offlead difficulties is disclosed in U.S. Pat. No. 2,042,460 issued to M. J. Fykse on June 2, 1936. In this patent, a sheave is rotatably carried by a sheave pin which is supported at the lower end of a yoke. The yoke is mounted for swinging motion on trunions disposed on a transversely extending pin at a position which is equidistant between the side plates of the boom. This arrangement has been quite satisfactory, but the present invention is intended to improve upon the Fykse construction.
Other boom point sheave assemblies which allow the sheaves to move towards the plane of the hoist ropes during offlead are known to those in the art. For example, in U.S. Pat. No. 3,757,959 granted to Thompson et al. on Sept. 11, 1973, a pair of sheaves are swivelly mounted on a shaft on a ball portion formed at its midpoint while the shaft is fixed at each end to the outer end of the boom. Since the sheaves are mounted on the enlarged ball portion, conventional sheaves cannot be employed in this assembly, and sheaves having large internal diameters and large bearings must be used. In U.S. Pat. No. 3,684,245 issued to Marichev et al. on Aug. 15, 1972, a transversely extending sheave-supporting shaft is connected at each end to the boom. A sheave is mounted between the sides of the outer end of the boom, and on a pin having a longitudinal axis upwardly disposed with respect to the central axis of the boom. The shaft in this patent also is fixed to the outer end of the boom, with the sheave mounted at its center. In both of these prior art arrangements, heavy bending loads can be imposed on the shaft. If the sheave shafts are increased dimensionally to accommodate heavy bending loads, structural weight is added to the boom point.